I would think welding would be the easy part (though I am not a welder) and forming the parts the more challenging. I used to build aircraft, and the challenging thing was forming the parts; of course most all we did was aluminum. But getting the parts shaped is essential to assembly, as I am sure you are well aware. That is my initial thoughts for what they are worth. I think it would be great if you can do it though! And I would enjoy hearing how you progress if you are willing.
BTW, I agree with the desire for metal instead of plastic mags. I really dislike the plastic mags for my Howa mini-action. I am hoping somebody will make metal ones some day... And it is the pits to have a really good gun that is kinda goofed up by plastic mags like you are dealing with.